Data read from a disk drive is typically received serially by the peripheral controller. The data is synchronized and framed by the controller. A command from the host computer to read data will include information on the drive and head selected as well as cylinder and sector information, each sector of data stored on the disk contains identification information as well as data. The ID field contains cylinder head and sector bytes, individually identifying the sector.
When the ID field has been framed, the controller compares the cylinder byte of the sector being read with the cylinder byte of the commanded sector address. A positive comparison of the cylinder will result in comparison of the head byte with the commanded head information. A positive comparison of the head will then result in a comparison of the sector number byte until the proper sector is located. Once the sector byte read by the controller matches the commanded sector byte, the data field will be read from the disk for transfer to the host computer.
If the read operation is initiated when the head is located at a point on the track displaced from the commanded initial sector address, the data from the track is not read until the commanded initial sector is reached and a sector byte compare is obtained. This reduces the potential speed of data transfer from the disk.
In addition, where data sectors are interleaved on the disk track, reading of data sectors from the track is interrupted since each sector in sequence must be located and compared before data is read from the sector. As a consequence, several revolutions of the disk may be required to obtain a full track of data from the disk drive.